Is Sugar Water Good For Plants

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Robert Pavlis

Is sugar water good for plants? This is a very interesting question and one that leads to all kinds of fun facts about plants.

Even more interesting is how this myth plays out on the internet. In most cases, popular gardening sites promote the myth, but in this case they tell you NOT to use sugar water for plants, except in special cases. They then give a variety of reasons why sugar water is bad for plants, but these explanations are mostly wrong. They are creating new myths with their explanations!

Many of these writers ignore basic plant biology or are unaware of the correct facts. I’m going to try and sort this thing out for you.

Is Sugar Water Good For Plants
Is Sugar Water Good For Plants

Should You Use Sugar Water For Plants?

The right amount of sugar will be beneficial to plants. The problem for the gardener is to know what the “right” amount is. Too little and it is a waste of time. Too much and you harm your plants.

Plant Science for Gardeners by Robert Pavlis

A well grown plant does just fine without a sugar source. I think it is better to focus on growing plants correctly, rather than experimenting with sugar amounts. However, I might start adding sugar to cut flowers.

If you would like to try sugar, read the following sections.

What is Sugar?

The term “sugar” can be misleading. To the layperson, sugar is the white stuff we sprinkle on our cereal and its chemical name is sucrose.

The term sugar is also used to describe a family of compounds that are carbohydrates (contain carbon, hydrogen and oxygen) and their names often end in “ose”: glucose, dextrose, fructose, maltose, lactose, etc. Some of these sugars are simple sugars that contain one sugar subunit and are called monosaccharides. When two of these join together they are called disaccharides and table sugar is an example of a disaccharide. Sucrose is a combination of glucose and fructose.

Sucrose is made up of glucose and fructose
Sucrose is made up of glucose and fructose, source Nutrients review

You probably know that plants use photosynthesis to make “sugar”, but which sugar? To understand this better have a look at my book Plant Science for Gardeners. Plants use photosynthesis to produce glucose and then convert glucose to sucrose because it is more soluble in water and easier to transport around the plant. Sucrose is also a more efficient way to store energy – it is a better type of battery than other types of sugars.

What is Osmotic Pressure?

Some chemicals are able to dissolve in water and when this happens it changes the physical properties of water. One of these properties is the osmotic pressure. Pure water has a low pressure of zero and dissolving chemicals in it increases the osmotic pressure. A sugar solution has a higher osmotic pressure than water. A concentrated solution has a higher osmotic pressure than a dilute  solution.

Cells have special membrane layers around them which are semi-permeable. In simple terms this means that water can move freely through the membrane but many other compounds can’t. Water always flows from an area of low pressure to one of high pressure. So if the osmotic pressure is higher inside the cell than outside the cell, water will flow into the cell. Plants take full advantage of this in their roots. The leaves make sugars and pass them down to the roots. This makes the water inside the roots sweeter and increases its osmotic pressure. This causes water to flow from outside the root (area of low osmotic pressure) to inside the root.

Is Sugar Water Good for Plants?

There is no single answer which is one reason many discussion on this topic fail. To answer the question you have to first define the concentration of sugar in the water.

At very low concentrations, the sugar will have limited impact on anything. It might help the plant a bit but it won’t harm it.

High concentrations affect plants quite differently. High sugar solutions near roots will have a higher osmotic pressure than inside the root. This causes water to flow out of the root instead of in, dehydrating the plant and killing it.

This is no different than adding too much synthetic fertilizer to soil. The fertilizer dissolves in the soil water, increasing the concentration near roots, which creates a higher osmotic pressure outside the plant. Water then flows out of the roots and the plant dies.

High osmotic pressure also kills bacteria in the same way and is a partial explanation why jams and honey don’t spoil without refrigeration.

Many online sites provide a formula for making sugar water but none of the ones I reviewed specified how much to add to the soil. Without that information you have no idea if it is too much sugar. Suggested mixtures range from 0.5 tabs/quart to 1/4 cup/4 cups (8 to 60 ml/l).

It is perfectly clear from our understanding of osmotic pressure that a high amount will kill plants. So the question we need to ask, and the one I will discuss in the rest of this post; is sugar water at lower doses beneficial to plants?

Do Plant Roots Absorb Sugar?

I will use the term “sugar” in the rest of this post to mean table sugar, namely sucrose.

One of the main arguments against adding sugar to plants is the claim that “plants do NOT absorb sugar through their roots”. But that is not true. Plants can absorb sucrose through the roots.

Plants make sucrose and use it as an energy source. If they can absorb it through their roots adding it to soil should benefit the plant, after all it does not mater if the sucrose came from the grocery store or was made in the leaves of a plant.

Plants expel sugar through their roots in the rhizosphere to attract microbes and they can reabsorb any excess sugar.

YouTube video

Sugar Activates Bacteria

Sugar is a great energy source for bacteria in the soil. They gobble it up and start reproducing. All this microbial activity is claimed to be good for plants and it is in some ways, but it also harms plants.

Higher microbe activity means that more nutrients are extracted from the soil and as the bacteria die these nutrients become available to plants. This is the positive aspect.

However, in order for microbes to grow they also need oxygen and nitrogen. Both of these are removed from soil decreasing the amount available to plants. I am not sure if the decrease in oxygen is significant enough to affect plants, but a lack of nitrogen can certainly be a problem.

What happens when the sugar is used up? The above chart shows what happens to the bacteria population when sugar is added to soil. There is a rapid increase in population until the bacteria use up all of the sugar. Suddenly, many bacteria die from lack of food. For a while the living can consume the dying, but in fairly short order the population is back to the starting point.

Is this a benefit to plants? There is some short term benefit from extra nutrients and some short term harm do to a lack of nitrogen. The net effect is that there is not much benefit to plants unless the sugar is constantly added to maintain a higher population and extra nitrogen is available.

Sugar Affects Flowering

Sugar is used by plants as a signaling molecule that regulates a variety of internal biological processes and one that is of direct interest to gardeners is flowering. When a plant enters the flowering phase there is an increase in sugar level near the bud meristem. The extra sugar signals the bud to form and eventually flower.

However, this does not mean that adding sugar to a plant will make it flower. Flowering is a complex process that requires several triggers to be in place and the sugar level is just one of these triggers. Day length is another trigger and it also has to be right before bud development takes place.

The other thing that is important is the actual sugar level. Lower amounts of sugar initiate the growth of buds, but high levels inhibit the growth of buds.

It is hard to see how a gardener can water with sugar and get the “amount” right for flowering. Remember it is the internal concentration that matters – not the amount you apply to soil. Sugar may be useful in plant production where a nursery controls the sugar level and timing of the application.

Does Sugar Affect Crop Yield?

A study in 2013 looked at sugar application to corn and soybeans and found no increase in yield. Another study sprayed various types of sugar on soybean crops and found no yield benefit. It compared 4 different sugar sources including molasses which many gardeners claim is superior to other sources of sugar. The sources studied were:

  1. Granulated cane sugar (100% sucrose)
  2. High fructose corn syrup (11 g glucose/fructose per 30 mL)
  3. Molasses (28 g sucrose per 30 mL)
  4. Blackstrap molasses (26 g sucrose per 30 mL)

In this study molasses was not better than the others, nor was it better than water.

A simple experiment growing radish seeds with and without sugar found they grew better with no sugar.

Using Sugar on Cut Flowers

Commercial cut flowers are harvested before they are fully developed and to develop properly they need carbohydrates. In uncut flowers these carbohydrates are supplied by the rest of the plant, but that can’t happen once they are cut. Sugar added to the water can provide the needed energy to fully develop the bloom and most flowers benefit from the addition of 2% sugar. Some, like gladiolus benefit from higher amounts (4-6%) and yet other plants (zinnias and coralbells) sustain damage at anything over 1%. Chrysanthemums and China asters, do better with no sugar added.

A 1% sugar solution contains 10 g/l (2 tsp/quart).

A sugar solution will encourage the growth of bacteria so you should also add a biocide to any sugar solution (50 ppm household bleach, 1 ml/l).

YouTube video

Sugar Water for Christmas Trees?

It won’t keep a Christmas tree fresh longer than just plain water.

Some Common Claims About Sugar Water

In this section I will list some claims I found on common gardening blogs and discuss their accuracy.

Sugar is a Good Fertilizer

Sugar contains carbon, oxygen and hydrogen. It contains none of the nutrients plants get from soil and is therefore not a fertilizer.

Sugar Provides Plants with Nutrients

Based on the previous statement you might think that this is false, but it is at least partially true. Sugar does not provide nutrients directly, but it can provide them indirectly. Microbes are better at extracting nutrients from soil than plants, especially for things like phosphate. As microbes die they make these nutrients available to plants. Microbe activity also lowers soil pH which makes some of the metallic nutrients, like iron, more available to plants.

Brown Sugar is Better Than White Sugar

This is wrong for two reasons.

First of all, plants benefit from sucrose. The sucrose in both white and brown sugar is identical.

Secondly, most commercial brown sugar is made by taking white sugar and adding some molasses to it. Brown sugar is essentially white sugar colored with molasses. Unrefined brown sugar is sugar that has been refined to a lessor degree, but it is still mostly sucrose.

Heavy Metals are More Readily Available and Will Harm Plants

The microbial growth mentioned above results in a lower pH which makes certain nutrients, including heavy metals, more available. The amount of these in soil is usually so small that this will not have a significant effect in most soils.

Sugar Hydrates Plants in a Drought

I have no idea where this idea came from but it might be a super-extrapolation of the way sugar controls things in plants. Sugar does activate a number of metabolic functions inside a plant including the activation of stress-inducible genes. But there is a big difference between sugar levels in a plant and sugar added to soil.

The level of soil water during drought conditions is very low. As levels drop the sugar in the soil will become more concentrated which causes an increase in osmotic pressure. That won’t hydrate a plant – it does the opposite. Using sugar water on a drought stressed plant might help because you are adding water, but it won’t be due to the sugar.

Alternate Pure Water with Sugar Water

Some of the advice online suggests watering with sugar water and then using pure water in subsequent waterings. It is claimed that, “the pure water will dilute and keep the amount of sugar to a safe level”. But if the original sugar water was at safe level, and bacteria consume excess sugar, why would you need to dilute it? None of this makes any sense.

If sugar water, at prescribed concentrations, is good for plants then you should be able to use it with every watering.

Adding Sugar to Soil Makes Tomatoes Sweeter

Tomato flavor is determined by the environment and genetics. Adding sugar to the soil will not make them sweeter.

Sugar Water Controls Weeds

The claim is that sugar encourages roots to seek nitrogen in soil which depletes the nitrogen level and in turn slows down plant growth in weeds.

Assume for a minute that sugar does encourage roots to find more nitrogen. Why would sugar only have this effect on the plants we want to grow and not on the weeds? This is a mistake in logic I see all the time in gardening circles. For some reason our precious plants have a different biology than weeds! I have news for you – weeds DO NOT KNOW they are weeds.

Roots are always looking for nitrogen and it is the nutrient that is usually in short supply. I see see no reason why sugar would accelerate this process? In fact, sugar may actually stimulate nitrogen-fixing bacteria which would increase nitrogen levels.

Sugar Water is Good for Dying Plants

The claim is that sugar water is “wonderful for boosting dying plants but it should not be used for every day watering”.

So sugar helps a dying plant but not a healthy plant? You’ve got to love such illogical claims that don’t even try to explain why they might be true – they just are!

Plants Use a Different Sugar

It is claimed that “plants do not metabolize sugar as humans do and the sugars they produce (glucose) have a different make-up to the polysaccharides of our store-bought sugar”.

Plants and animals do metabolize things differently but when it comes to converting sugar to energy, they both use the same set of reactions known as “glycolysis”, the splitting of glucose. It is even used by microbes.

Store bought sugar is sucrose, a disaccharide, not a polysaccharide (e.g. starch).

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

23 thoughts on “Is Sugar Water Good For Plants”

  1. My Mom use to get expired dextrose ivs from the Fire Department when she worked for them, she used the Dextrose on the indoor plants and they grew like weeds. A little Pothos start would be ten feet long in a month, she had the Pothos going around the living room twice roughly 70 feet, her Ficus grew 6 feet in a year to about 8 feet tall inside the house.

    Reply
    • I thought I would have a quick look at some research.

      “Brix refractometry was not an applicable method to use for determining resistant sorghums to sugarcane aphids.” – Brix refractometry was not an applicable method to use for determining resistant sorghums to sugarcane aphids.

      This paper has an interesting discussion about how consultants sell the idea of brix vs feeding insects – but found no correlation. “Preliminary findings such as these appeared to
      contradict theproposedgrapeleaf sap Brix/leafhopper
      theory. There appeared tobevirtually noempirical
      evidence tosupportsucha predictive Brix/leafhopper
      relationship, although a number offarmers and agricul
      turalconsultants havedescribed corroborating anecdotal
      observations”

      “Results compiled during this two-year empirical
      study of eight San Joaquin Valley vineyards (including
      five organic operations) provide noconsistent support
      for the claimed predictive relationship between
      grapeleaf sap Brix levels and leafhopper densities”

      https://scholarworks.calstate.edu/downloads/hq37vp59c

      Reply
  2. i look after a bowling green in a park, my problem is squirels
    [grey] they bury food [nuts] in the green leaving holes all over the place. is there some sought of deterent.

    Reply
  3. The mechanism as told to me is that the mass planting of Marigolds attracts the nematodes from the soil and when the Marigolds mature and die and are then removed, the nematodes go with them. Or at least the infestation is greatly reduced. Perhaps it is an ongoing treatment.
    My advisor has taught horticulture at technical college level so I am relying on her expertise. I will check to see if she has an published sources – or if her view is just based on “experience” ie hypothetical. But i am in “test but verify” mode, looking forward to seeing if the Marigold roots come out nobbly with nematode swellings. I am a growing season away I fear. She recommends removal rather than turning them in.

    Reply
  4. Check the inputs of the farmers who grow worlds record corn and soy beans. They add fulvic acid and a little sugar, sometimes molasses and sometimes a manufactured sugar. They almost always add it with a foliar application of nitrogen and microbes. It has been proven in the field by farmers. This addition to crop inputs has increased the worlds record in corn from 424 bu/acre in 2012 to 616 bu/acre in 2020.

    Reply
    • 1) Science is NOT proven by farmers in the field.
      2) You said “They add fulvic acid and a little sugar” – so they don’t know which ingredient actually made a difference. This is one reason why #1 is true.
      3) Lots of things, including new hybrids have contributed to crop increases over time – that does not mean sugar had anything to do with it.

      Reply
  5. I use your garden myth books for reference and am always pleased to see a new article. Really appreciate your analysis and experience. Thanks!

    Reply
  6. A question not related to sugar water. I have two semi dwarf peach trees purchased as root stock. One came with a 2” trunk, (2” from the graft to the first branch) and the other is an 18” trunk. The 2” trunk has been in my garden 2. 5 years the 18” trunk 8 months. They are both 7 feet tall but the short trunk peach looks like a bush. My question is; will the short trunk ever get taller and look like a tree? I have a semi dwarf grapefruit with the same problem.

    Reply
    • In your article you have the following two sentences that appear to contradict each other. Can you please clarity?
      Under: SugarActivates Bacteria- “However, in order for microbes to grow they also need oxygen and nitrogen. Both of these are removed from soil decreasing the amount available to plants.”
      Under: Sugar Water Controls Weeds- ” In fact, sugar may actually stimulate nitrogen-fixing bacteria which would increase nitrogen levels.”

      Reply
  7. A lot of cannabis growers add black strap molasses to the water they water their plants with. Is this a waste of time and/or harmful to the plants?

    Reply
  8. Should have added that the follow-up is to plant French Marigolds in the barrow, let them mature and die off then dump the plants which are expected to have attracted the little blighters and (more or less) cleared the soil – well, planting medium which is a good mix of sand, soil and organic matter, among the best consistency of all my container plantings.

    Reply
  9. I have been advised to use molasses (1 cup in a 9 litre bucket of water x 3) to deal with nematodes – and I have just administered this to soil in an old wheel barrow that I use for ornamentals and herbs – my Italian Parsely was lousy with root knot nematodes. I am doing this as much as an experiment as anything – gardener’s curiosity – rather than just dumping the soil (gradually) in the rubbish bin. I will not be cooking soil in the oven or leaving it under plastic to cook in the sun. Your chemistry analysis (osmosis generally) seems to align with the advice I have been given.
    I’d appreciate your viewpoint as to likely success please.

    Reply
    • Why not ask the one who gave you this advice – they should have some studies to show it works.

      Does molasses harm nematodes? I doubt it.

      Reply

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